1. Field of the Invention
The present invention relates to a method of accurately punching holes into a moving web of a thermoplastic film material, and more particularly, punching holes into the web through the intermediary of a rotary serrated tube punch. Moreover, the invention also relates to accurately punching holes into an advancing web of thermoplastic film material utilizing a novel rotary serrated tube punch.
In the production of various types of articles which are basically formed from a thermoplastic film material, such as polyethylene film or the like; for instance, articles such as plastic bags employed in supermarkets or plastic garbage disposal or trash bags, it is frequently necessary to provide holes in a web of the thermoplastic film material employed for the high-speed series production of the bags, which must be punched through at a high degree of accuracy with respect to the location of the holes in the film web and repetitions in successive hole patterns. Thus, for instance, with regard to plastic bags onto which tapes are to be applied and which are used for closing trash bags in the manner of a drawstring, and into which there must be provided holes in order to afford a consumer access to the tape for closing the bags, the inability of presently employed production equipment in providing accurately positioned and dimensioned holes necessitates wider border edges to be formed on the film web for the receipt of such tapes, with the result of requiring greater amounts of plastic material in the formation of the bags thereby rendering current manufacturing methods less than optionally economical.
Thus, a need has arisen in industry for methods and apparatus for accurately punching holes of various sizes and configurations through webs of thermoplastic film material, in which the webs are conveyed in a continuous mode between processing stations so as to afford optimized production rates and extremely high degrees of manufacturing accuracy, without the need to stop the continual advance of the film web during the hole punching procedure.
Heretofore, one particular method of punching holes through polyethylene film webs contemplated the use of hole punching apparatus incorporating a tube forming a die punch having a cutting edge consisting of serrations or sharpened teeth facing towards the film. For example, a punching apparatus of that type is disclosed in Adams, et al. U.S. Pat. No. 3,580,120. The serrated cutting edge of the punch is then pressed through the film web while the latter is in a stationary condition during the intermittent advance of the web and with the web being supported externally of the die punch in order to maintain the necessary tension on the film web. Thus, in order to employ a serrated punch for punching holes into a film web, it is necessary to either advance the web only intermittently in order to prevent it from moving while the hole is being punched, or to reciprocate the punch so as to match the web speed when the latter is continually advanced, or to press the punch through the moving web and, as a result, produce a relatively ragged and inaccurate hole. Consequently, the prior art hole punching methods and apparatus are either complex in construction and/or cumbersome in use, and fail to provide the required accuracy in forming punched holes in a continuously advancing web of thermoplastic film material.
2. Discussion of the Prior Art
Although rotary film web or sheet material punching devices are known in the technology, none of these devices are analogous to the arrangement pursuant to the present invention and to implementing the hole punching method as disclosed herein.
Stoop U.S. Pat. No. 3,680,419 discloses a rotary punch device in which a hollow rotating punch has a projecting cutting edge which is adapted to cut holes into a paper web advanced over an anvil roll, and wherein the chips which are punched out of the paper web aspirated through the punch by a vacuum. This prior art device, however, fails to provide for tensioning a thermoplastic film web about the circumference of an anvil roll and the punching of holes therethrough by a serrated cutting edge on a punch analogous to that of the present invention, and thus would not be capable of providing the desired degree of accuracy in the punching of holes into a continuously advancing web of a thermoplastic film material.
Leroy U.S. Pat. No. 4,480,516 discloses a rotatable cutting device contacting an anvil or counter roll over which a film web is advanced by means of a cutter possessing a cutting edge for severing the web into predetermined sections. This does not allow for the formation of holes in an accurate manner analogous to that afforded by the device and method pursuant to the present device.
Finally, Helm U.S. Pat. No. 3,728,918 discloses a rotary panel cutter through the intermediary of which sections are cut out of a continuously advanced film web. However, there is no disclosure of a serrated cutter engaging into an opening in an anvil roll to punch accurately-sized holes into a film web analogous to that of the present invention.